<!DOCTYPE html>  
<html>  
  
    <head>  
        <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />  
        <title>文本框根据输入内容自适应高度</title>  
        <style type="text/css">  
            h2 {  
                text-align: center;  
                margin: 50px auto;  
            }  
              
            #textarea {  
                display: block;  
                margin: 0 auto;  
                overflow: hidden;  
                width: 550px;  
                font-size: 14px;  
                height: 18px;  
                line-height: 24px;  
                padding: 2px;  
            }  
              
            textarea {  
                outline: 0 none;  
                border-color: rgba(82, 168, 236, 0.8);  
                box-shadow: inset 0 1px 3px rgba(0, 0, 0, 0.1), 0 0 8px rgba(82, 168, 236, 0.6);  
            }  
        </style>  
        <script>  
            /**  
             * 文本框根据输入内容自适应高度  
             * @param                {HTMLElement}        输入框元素  
             * @param                {Number}                设置光标与输入框保持的距离(默认0)  
             * @param                {Number}                设置最大高度(可选)  
             */  
            var autoTextarea = function(elem, extra, maxHeight) {  
                extra = extra || 0;  
                var isFirefox = !!document.getBoxObjectFor || 'mozInnerScreenX' in window,  
                    isOpera = !!window.opera && !!window.opera.toString().indexOf('Opera'),  
                    addEvent = function(type, callback) {  
                        elem.addEventListener ?  
                            elem.addEventListener(type, callback, false) :  
                            elem.attachEvent('on' + type, callback);  
                    },  
                    getStyle = elem.currentStyle ? function(name) {  
                        var val = elem.currentStyle[name];  
                        if (name === 'height' && val.search(/px/i) !== 1) {  
                            var rect = elem.getBoundingClientRect();  
                            return rect.bottom - rect.top -  
                                parseFloat(getStyle('paddingTop')) -  
                                parseFloat(getStyle('paddingBottom')) + 'px';  
                        };  
                        return val;  
                    } : function(name) {  
                        return getComputedStyle(elem, null)[name];  
                    },  
                    minHeight = parseFloat(getStyle('height'));  
                elem.style.resize = 'none';  
                var change = function() {  
                    var scrollTop, height,  
                        padding = 0,  
                        style = elem.style;  
                    if (elem._length === elem.value.length) return;  
                    elem._length = elem.value.length;  
                    if (!isFirefox && !isOpera) {  
                        padding = parseInt(getStyle('paddingTop')) + parseInt(getStyle('paddingBottom'));  
                    };  
                    scrollTop = document.body.scrollTop || document.documentElement.scrollTop;  
                    elem.style.height = minHeight + 'px';  
                    if (elem.scrollHeight > minHeight) {  
                        if (maxHeight && elem.scrollHeight > maxHeight) {  
                            height = maxHeight - padding;  
                            style.overflowY = 'auto';  
                        } else {  
                            height = elem.scrollHeight - padding;  
                            style.overflowY = 'hidden';  
                        };  
                        style.height = height + extra + 'px';  
                        scrollTop += parseInt(style.height) - elem.currHeight;  
                        document.body.scrollTop = scrollTop;  
                        document.documentElement.scrollTop = scrollTop;  
                        elem.currHeight = parseInt(style.height);  
                    };  
                };  
                addEvent('propertychange', change);  
                addEvent('input', change);  
                addEvent('focus', change);  
                change();  
            };  
        </script>  
        <script async src="http://c.cnzz.com/core.php"></script>  
    </head>  
  
    <body>  
        <h2>文本框根据输入内容自适应高度</h2>  
        <textarea id="textarea" placeholder="回复内容"></textarea>  
        <script>  
            var text = document.getElementById("textarea");  
            autoTextarea(text); // 调用  
        </script>  
    </body>  
  
</html>